The pipeline has a predetermined inner diameter in a predetermined flow direction and includes: At least two ultrasonic transducers, which emit, or transmit, and/or receive ultrasonic measuring signals along defined sound paths; and a control/evaluation unit, which ascertains volume and/or mass flow of the medium in the containment using the ultrasonic measuring signals based on the travel-time difference principle. The measured medium can be gaseous or liquid. The flow measuring device is either a clamp-on device or an inline device installed into the pipeline.
Ultrasonic flow measuring devices of the above-described kind for ascertaining volume flow by means of the so-called travel-time-difference method are often applied in process and automation technology. Especially clamp-on flow measuring devices have the advantage of enabling ascertainment of volume flow in a containment, e.g. in a pipeline, contactlessly, i.e. without needing to contact the medium. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478, DE 43 35 369 C1, DE 298 03 911 U1, DE 4336370 C1 or U.S. Pat. No. 4,598,593.
In the case of both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated at a predetermined angle into, respectively received from, the containment in which the medium is located. The positions of the ultrasonic transducers on the measuring tube, in the inline case, and on the pipeline, in the clamp-on case, depend on the inner diameter of the pipe, or tube, and on the velocity of sound in the medium being measured.
For a clamp-on measuring device, additionally the application parameters, wall thickness of the pipeline and velocity of sound in the material of the pipeline, must be taken into consideration.
Essential component of an ultrasonic transducer is usually a piezoelectric element. The ultrasonic measuring signals produced or received by the piezoelectric element are led into and out of the interior of the pipeline, or measuring tube, as the case may be, via a coupling wedge, or interfacing shoe, and, in the case of a clamp-on measuring device, via the pipe wall.
Usually, in the case of both types of ultrasonic flow measuring devices, the two ultrasonic transducers are so arranged that the traversing sound paths are sent through the central region of the pipeline, or measuring tube, as the case may be. The ascertained flow measurement value thus reflects an average flow of the medium in the pipeline, or measuring tube. In many applications, especially in the case of flow measurements in pipelines of large nominal diameter, this averaging is, however, too inexact. Therefore, it is also known in the case of inline flow measuring devices to provide at the measuring tube a plurality of sensor pairs distributed over the periphery of the measuring tube, whereby flow information from various segmented, angular regions of the measuring tube can be made available. Of course, this solution is naturally relatively expensive, due to the increased number of sensor pairs.
Corresponding, multipath, or multichannel, arrangements for segmented flow measurement have not been introduced in the case of clamp-on flow measuring devices. Instead, in the case of clamp-on flow measuring devices, the sound paths always go through the central region of the pipeline, whereby the flow is provided integrated over all regions of the pipeline. In order, however, to obtain the flow profile exactly, it is necessary to direct the sound paths through predetermined segments of the pipeline, or measuring tube, as the case may be. If an aggressive medium is being measured, or if there is no possibility of using an inline flow measuring device, then other measurement principles must be resorted to, since known clamp-on flow measuring devices exhibit a relatively low, measurement accuracy.